1. Field of the Invention
The present invention relates to a process for scrubbing porous hollow fiber membranes in a hollow fiber membrane module(s) disposed in a hollow fiber membrane filter column, which is used to treat condensate water obtained in a nuclear or thermoelectric power plant as well as industrial waste water or the like.
2. Prior Art
In general, a hollow fiber membrane filter column comprises a number of hollow fiber membrane modules including a large number of porous hollow fibers bundled together therein. The hollow fiber membrane modules are juxtaposed with each other generally in a state of being hung from a module support laterally provided in the filter column. The filtration of raw water is effected by feeding raw water into a lower compartment partitioned with the module support to allow the raw water to permeate through the pores of the hollow fiber membranes from the outsides of the hollow fibers to the insides thereof. Fine particles in the raw water are trapped on the outer surfaces of the hollow fibers, while filtrate obtained on the insides of the hollow fibers is gathered in an upper compartment partitioned with the module support and is then drained out of the filter column.
When the differential pressure, or pressure drop, across the filter column is increased to some level through the step of such filtration, bubbles are charged into water around the outer surfaces of the hollow fibers dipped in water to cause agitation of the water with which the hollow fiber membranes are vibrated to exfoliate the fine particles trapped on the outer surfaces of the hollow fibers. This step, which is called a "scrubbing step," is followed by the drain step of discharging from the lower compartment the waste water resulted from the scrubbing and containing the exfoliated fine particles. Thus, the operation of the filter column involves the filtration step, the scrubbing step and the drain step.
Additionally stated, the backwash step of inversely flowing washing water from the insides of the hollow fibers to the outsides thereof may sometimes be taken before, during or after the scrubbing step.
As means for charging bubbles such as air bubbles around the outer surfaces of the hollow fibers, there can be mentioned release of a gas such as air on the lower side of the hollow fiber membrane modules to charge air bubbles into the insides of the hollow fiber membrane modules, and injection of a compressed gas such as compressed air into the central bores of the hollow fibers to release the gas through the pores of the hollow fiber membranes to form bubbles around the outer surfaces of the hollow fibers. The former is employed in most cases where use is made of hollow fiber membranes having such fine pores as to hardly allow a gas to pass therethrough, while the latter is employed in most cases where use is made of hollow fiber membranes having such fine membrane pores as to allow a gas to easily pass therethrough.
Thus, the filter column using hollow fiber membranes is basically operated by repeating the filtration step, the scrubbing step and the drain step. Accordingly, sufficient care is needed to ensure that continuation of filtration is not obstructed simply because fine particles trapped on the outer surfaces of the hollow fibers are accumulated to raise the differential pressure across the filter column.
Therefore, investigations, tests and developments have heretofore been made of the structures of hollow fiber membrane modules, the structures of filter coulmns, methods of washing hollow fiber membranes by scrubbing in particular, etc. with an eye to preventing fine particles from accumulating while trapped on the outer surfaces of hollow fibers.
The authors of the present invention have made efforts as well to develop a process for effectively washing hollow fiber membranes in hollow fiber membrane modules disposed in a filter column. They have found out, however, that, in some cases of treating such raw water containing fine iron oxide particles as condensate water obtained, for example, in a BWR nuclear power plant, the differential pressure across a hollow fiber membrane module, increased through the filtration step, is not reverted to the original one even when the module is subjected to the aforementioned scrubbing and backwash steps or even when those scrubbing and backwash steps are further followed by subjecting the hollow fiber membrane module to the step of washing hollow fiber membranes with an acid to dissolve and remove therefrom the iron oxides adhering to the outer surfaces of the hollow fiber membranes.
With a view to looking into the causes of the foregoing phenomenon, the authors of the present invention have made various investigations and experiments, as a result of which the following have been elucidated:
(1) A cause for non-reversion of the differential pressure is lowering of the water permeability of the hollow fiber membranes themselves, not due to compaction and/or crush of the hollow fiber membranes owing to the pressure differential between the outside and inside thereof.
(2) Only the outer surface portions of the hollow fibers are, so to speak, roughened such that most pores originally present are occluded to considerably decrease the number of open pores all across the hollow fiber membranes.
(3) The roughened state of the outer surface portions of the hollow fibers does not change even when the hollow fibers are washed with a chemical such as an acid, an oxidizing agent or a reducing agent.
(4) The non reversion of the differential pressure is not a result of deterioration of such physical properties of the hollow fibers as manifested in terms of lowerings of their mechanical strengths such as tensile strength, tensile elongation and crushing strength.
As a result of further investigations and experiments, the following have futher been elucidated.
(5) The outer surface portions of the hollow fibers are roughened through collision thereagainst of fine particles such as iron oxide particles.
(6) The collision of the fine particles against the outer surfaces of the hollow fibers occurs when the fine particles exist between the hollow fibers being vibrated.
(7) The above-mentioned collision is liable to occur most frequently in the step of scrubbing the hollow fiber membranes.
(8) The fine particles such as iron oxide particles which are relatively coarse and hard (hereinafter referred to as "relatively coarse, hard, fine particles") have been found out to be predominantly involved in the roughening of the outer surface portions of the hollow fibers.
(9) In an environment where fine iron oxide particles contained in such raw water as condensate water having a low iron oxide particle content and obtained in a BWR nuclear power plant in particular are trapped on the outer surfaces of hollow fibers over a long period of time while iron ions existing along with the fine iron oxide particles are adsorbed and deposited in the layer of the trapped fine iron oxide particles, fine iron oxide particles, even if very fine, in raw water are grown into firm granules on the outer surfaces of the hollow fibers. Therefore, fine iron oxide particles exfoliated off the hollow fiber membranes in the scrubbing step in the case of the above-mentioned environment naturally include relatively coarse, hard, fine particles as mentioned above.
(10) Such relatively coarse, hard, fine particles are exfoliated in the initial stage of the scrubbing step.
The present invention has been made based on these findings.
Accordingly, an object of the present invention is to provide a process for scrubbing porous hollow fiber membranes in at least one hollow fiber membrane module generally disposed in a filter column, according to which the influence of relatively coarse, hard, fine particles on hollow fibers to roughen the outer surface portions (hereinafter often referred to in brief as the "outer surfaces") thereof can be suppressed as much as possible without adversely affecting exfoliation of fine particles trapped on the outer surfaces of the hollow fibers.